Who can help me understand what’s hidden under my apartment?

Hi all!

I live in a duplex apartment (floors 0 and 1) in Brussels.

Any guesses as to what the dotted squares/rectangles with a dotted circle in them could be? The one located in ‘kelder 1’ is where a toilet is currently installed and which is connected to the sewage (obiviously). The one in ‘kelder 2’ doens’t seem to be connected to any sewage system at all according to the drawings.

I’m asking because we’re confronted with a sewage-like smell popping up every now and then. The problem seems to be more prevalent in ‘kelder 2’ and serious during winter/fall than during summer/spring. It gets pretty severe at times.

How could we be sure about the cause of this?

Thanks in advance!

by pillouter